1. Field of the Invention
The present invention relates in a general manner to a method for determining at least one geometric/physiognomic parameter associated with the mounting of an ophthalmic lens in a spectacle frame worn by a user.
2. Description of the Related Art
The manufacture of a corrective spectacle lens comprises, on the one hand, the optical design and shaping of the refractive surfaces of the lens and, on the other hand, the adaptation of the lens to the chosen frame.
The present invention relates to the measurement, on the face of the user, of geometric/physiognomic parameters taking into account the fitting configuration of the spectacles on the user's face. These parameters are likely to be used in the two steps of manufacture of a corrective lens so that the lens finally performs the corrective optical function for which it was designed and prescribed. This notably involves the interpupillary distance, the height of the pupils of the users eyes in relation to the lower edge of the frame, and/or the pantoscopic angle formed by the general plane of the frame or the lens in relation to the perpendicular.
In a known manner, it is possible to determine these parameters on the basis of one or more captured images of the users head.
It is also useful to determine the optical behavior of the user, for example by determining the orientation of the gaze and the head of the user when the latter follows a moving point with his eyes, for example during a movement of the head from left to right and/or from right to left. A video acquisition of a plurality of images during the movement of the head is particularly suitable for this purpose.
The determination of the aforementioned parameters entails the identification on the captured image of the image of at least one reference element disposed on the face of the user and having at least one predetermined geometric characteristic.
It is then possible to determine the looked-for geometric/physiognomic parameter by comparing the geometric characteristic of the image of the reference element and its corresponding real geometric characteristic.
However, this method can only be carried out if the identification of the reference elements on the captured image is possible.
A positioning of the user backlit in relation to the image capture device can result in the capture of an unusable image, as it is too dark to allow identification of the reference element.
Moreover, if the user's head is illuminated at least partially by daylight, the illumination of the user's head may vary, either according to the course of the sun in the sky, or, in the case of a dynamic measurement, according to the position of the user's head in relation to the light sources illuminating it. Thus, the conditions of illumination of the user's head can be variable.
The situations in which the reference elements of the user's head appear too dark to be identified precisely on a captured image are therefore multiple.
A first solution to this problem is to place the image capture device in a location where the illumination is perfectly controlled and constant. The main disadvantage of this solution is that it limits considerably the locations that are favorable for the installation of the device allowing the method to be carried out.
The optician may then be forced to place the image capture device in a sealed room. He cannot then place it in a shop window, even though that offers advantages from a commercial point of view.
A different solution is to use an image capture device comprising automatic means for adjusting the brightness of the captured image. However, these means carry out an adjustment of the mean brightness of the entire image, which does not allow a usable image to be obtained in the case of a backlit image capture, for example.
Other known image capture devices comprise means for manual brightness adjustment. The brightness setting can then be adjusted before each image capture. However, this has the disadvantage of being long and tedious for the optician. Furthermore, this cannot be implemented in the context of a continuous image acquisition in video mode.